ORIGINS OF DEEP SEA POLYUNSATURATED FATTY ACIDS ASSESSED USING COMPOUND SPECIFIC STABLE ISOTOPES

The transfer of material and energy from the surface to the deep ocean via biotic interactions within ecosystems, known as the biological pump, controls, in part, atmospheric carbon dioxide uptake by the ocean, the cycling of elements and nutrients, and provides the main food supply for organisms living within the ocean interior. Lipid biomarkers are powerful tools for identifying carbon sources and transformations during particle cycling and flux. Polyunsaturated fatty acids (PUFAs) are essential fatty acids produced by phytoplankton and are transported to depth via the particle flux and biotic interactions. To determine whether there is also a significant deep ocean bacterial source of PUFAs, we analyzed the carbon isotopic (δ¹³C) signatures of PUFAs and other fatty acids in particulate organic carbon (POC) profiles collected by the Oceanic Flux Program (OFP) time-series in the Sargasso Sea. Saturated and polyunsaturated fatty acids (PUFAs) in the POC showed similar enrichment patterns, with lipids at the depth of the chlorophyll maximum (80 m) most depleted in ¹³C (between -28 ‰ and -26 ‰). There is a large ¹³C enrichment of 4 ‰ from 80 m to 500 m, followed by a smaller increase with depth in the deep water column. A branched 19 FA (presumably bacterially-sourced), not present in the upper water column, had a similar enrichment of 3 ‰ between 500 and 1500 m. Within the PUFAs, the 22:6ω3 and 20:5ω3 show an enrichment of ~2 ‰ from 3200 m to 4200 m, which was not observed in the 22:5ω3. The observed ¹³C enrichment with depth is consistent with isotopic fractionation during degradation in the water column and is opposite to an expected ¹³C depletion (based on published experimental studies) if PUFAs in the deep ocean POC are largely sourced from deep ocean bacteria. Overall, isotopic information on lipid biomarkers provides additional insights into the complex dynamics of changing community structure within the deep ocean and its role in carbon cycling.